1. Field of the Invention
The present invention relates to a process for the preparation of dialkyl carbonates by reacting carbon monoxide (CO) with alkyl nitrites in the presence of a heterogeneous platinum metal catalyst, the activity of the catalyst being maintained by the addition of small amounts of halogen.
Dialkyl carbonates are of general chemical and industrial importance. Thus, for example, diethyl carbonate is an excellent solvent in the medium boiling range. Dialkyl carbonates are also excellent carbonylating and acylating reagents. They are of great importance in the preparation of other carbonates, urethanes and ureas. Finally, on account of their high oxygen content, they are suitable as fuel additives for improving the knock rating of motor fuels.
2. Description of the Related Art
It is known to prepare dialkyl carbonates by reacting phosgene or alkyl chloroformates with alcohols.
There is an increasing interest, however, in superseding the use of the toxic phosgene or the intermediates derived therefrom, such as chloroformic acid esters, by other processes.
Particularly important processes here are those in which CO is reacted in the gas phase with alkyl nitrites on heterogeneous platinum metal catalysts.
Thus, Zeitschrift fur Katalytische Forschung (China) vol. 10(1) pp. 75-78 (March 1989) describes the reaction of CO and methyl nitrite on a PdCl.sub.2 -containing activated charcoal catalyst to form mainly dimethyl carbonate in addition to dimethyl oxalate.
In DE-OS (German Published Specification) 41 23 603, a high selectivity, based both on CO and on methyl nitrite, coupled with a high conversion, is achieved by using a heterogeneous palladium chloride catalyst with .gamma.-Al.sub.2 O.sub.3 as the support. However, to maintain the catalytic activity, gaseous hydrogen chloride must be added to the educt mixture in amounts of up to 1000 ppm (by volume). This can lead to corrosion problems if it is proposed to carry out this process on the industrial scale.